1. Technical Field
A method of forming an isolation film in a semiconductor device is disclosed which is capable of preventing concentration of an electric field on the top corners of the trench in the isolation film of a STI (shallow trench isolation) structure.
2. Background of the Related Art
In all of the semiconductor devices, the isolation films for electrically isolating various devices formed in the semiconductor substrate are formed. Conventionally, the isolation film is formed by means of LOCOS (local oxidation) process. In this case, however, a bird's beak occurs the corners of the isolation film. Due to this, there are problems that the electrical characteristics and degree of integration of the device are degraded.
As the degree of integration in the semiconductor device becomes higher, the isolation film is formed to have a STI (shallow trench isolation) structure that can minimize generation of the bird's beak at the isolation film while preventing generation of it.
A conventional method of forming the isolation film in the semiconductor device will be described by reference to FIG. 1A˜FIG. 1D.
Referring to FIG. 1A, a pad oxide film 102 and a pad nitride film 103 are sequentially formed on a semiconductor substrate 101. A photoresist is then covered on the pad nitride film 103. Next, exposure and development processes are implemented to form a photoresist pattern 104 in which an isolation region where the isolation film will be formed is defined. Thereby, the pad nitride film 103 in the region where the isolation film is to be formed is exposed.
By reference to FIG. 1B, the pad nitride film 103 in the isolation region is removed through an etch process. The pad oxide film 102 exposed below is then sequentially removed. Thereby, the semiconductor substrate 101 in the isolation region is exposed.
Referring to FIG. 1C, the semiconductor substrate 101 in the isolation region is etched by a given depth to form a trench 105. Next, the photoresist pattern (104 in FIG. 1B) is removed.
By reference to FIG. 1D, an insulating material layer (not shown) is formed so that the trench 105 is buried. Next, the insulating material layer on the pad nitride film (103 in FIG. 1C) is removed by means of a chemical mechanical polishing process. Also, the pad nitride film and the pad oxide film (102 in FIG. 1C) are sequentially removed by means of an etch process. Thereby, the insulating material layer remains only in the trench to form an isolation film 106 consisting of the insulating material layer.
The isolation film formation method described above will be examined. In case that the isolation film is formed to have the STI structure, the bird's beak does not occur. Therefore, it is possible to prevent degradation in the electrical characteristics and the degree of integration due to the bird's beak.
However, the most vulnerability in the isolation film of the STI structure is that the electric field is concentrated on a portion where the top corners of the trench (105a in FIG. 1C) are sharp-pointed. If the top corners of the trench are thus pointed, a gate oxide film is thinly formed at this portion, which increases the leakage current at this portion. The electric field is concentrated on that portion, which may change the threshold voltage of the transistor and cause defect in the device. Therefore, there is a problem that reliability of the device is degraded.
A method of preventing this is one by which the photoresist pattern is used as an etch mask without using the pad nitride film as the etch mask when the substrate is etched to form the trench. This method will be described in detail.
In FIG. 1C, in the state that the photoresist pattern remains in tact on the pad nitride film 103 without removing the photoresist pattern, as in FIG. 1B, if a polymerization reaction is generated using an etchant upon etch of the substrate for forming the trench, polymer is accumulated on the substrate at the corners of the isolation region while polymer is generated. As a silicon component and an etch selectivity ratio are different in the accumulated polymer, the accumulated polymer serves as an anti-etch film when the substrate is etched. Due to this, the corners of the isolation region on which polymer is accumulated is rarely etched compared to the center of the isolation film. Therefore, the top corners of the trench are made rounded.
However, this method could not exactly control the amount of generation of polymer. It is thus difficult to uniformly form the top corners of the trench in a rounded shape. Furthermore, in case that the photoresist pattern is used as the etch mask, it becomes difficult to etch the semiconductor substrate due to polymer occurring in the photoresist as the pattern size is integrated. Accordingly, it is required that the semiconductor substrate be etched with the photoresist pattern removed. However, when the pad nitride film is used as the etch mask with the photoresist pattern removed, carbon source for generating polymer is short. Therefore, it is further difficult to make rounded the top corners of the trench using polymer.